Emulsion and solution polymers find wide application as adhesives, binders and coatings. Unfortunately, many of these polymeric materials, especially those prepared predominantly from vinyl acetate, ethylene, vinyl chloride, or their mixtures, show inadequate resistance to water and other solvents in their everyday use. In particular, they experience substantial and unacceptable loss in strength in the presence of solvents such as perchloroethylene, methyl ethyl ketone and toluene. In addition, many of these polymers exhibit deficiencies in adhesion to the substrates on which they are used, for example vinyl acetate, ethylene or vinyl chloride polymers on glass, metal or polyester. These deficiencies are reduced, especially for relatively hydrophilic monomers, by the use of adhesion promoting or crosslinking comonomers and/or post-added crosslinkers.
Among types of widely used crosslinking materials are aminoplasts, especially N-methylolacrylamide, melamine/formaldehyde and urea-formaldehyde condensates. These materials have met substantial success because they are low in cost, highly compatible with aqueous emulsion systems, rapidly cured under acid catalysis and are substrate reactive in that, for example, they react with the hydroxyl groups of cellulosic materials. These crosslinking materials, however, suffer from two deficiencies: (1) the emission of low levels of formaldehyde during cure and subsequent use, and (2)inadequate adhesion to certain substrates, for example, metal, glass and polyester.
Many attempts have been made to overcome or minimize the first deficiency, especially after the potential carcinogenicity and irritant properties of formaldehyde became widely recognized.
To reduce the level of formaldehyde in emulsion products, the use of O-alkylated N-methylolacrylamides such as butoxymethylacrylamide or the use of about equimolar ratios of N-methylolacrylamide with acrylamide were introduced. These materials did not, however, totally eliminate the presence of formaldehyde.
U.S. Pat. Nos. 4,691,026, 4,663,410, 4,788,288 and 4,864,055 disclose self- and hydroxyl reactive formaldehyde-free cyclic hemiamidal and hemiamide ketal monomers and polymers formed from such monomers. The monomers can be incorporated into free radical addition polymers which undergo efficient acid catalyzed, thermally activated post-crosslinking with themselves or, alternatively, can react with active hydrogen-containing comonomers of the polymers and/or with groups on the substrate to which the polymer is applied. These materials were advantageous over prior crosslinking systems in that they provided for good crosslinking and adhesion promotion without the accompanying emission of formaldehyde.